Method for producing pulverized coal

ABSTRACT

Method for producing pulverized coal, the method comprising the steps of heating a drying gas, preferably an inert gas, in a hot gas generator ( 26 ) to a predefined temperature; feeding the heated drying gas into a pulverizer ( 20 ); introducing raw coal into the pulverizer ( 20 ), the pulverizer ( 20 ) grinding the raw coal into pulverized coal; collecting a mixture of drying gas and pulverized coal from the pulverizer ( 20 ) and feeding the mixture to a filter ( 34 ), the filter ( 34 ) separating the dried pulverized coal from the drying gas; collecting the dried pulverized coal for further use and feeding the drying gas from the filter ( 34 ) to a recirculation line ( 38 ) for returning at least part of the drying gas to the hot gas generator ( 26 ); and determining an oxygen level in the drying gas, preferably in the recirculation line ( 38 ), and comparing the determined oxygen level to a predetermined oxygen level threshold. According to a preferred embodiment of the invention, if the determined oxygen level is higher than a predetermined oxygen threshold, water is injected into the heated drying gas before it is fed into the pulverizer ( 20 ), the volume of water injected being calculated so as to reduce the oxygen level below the predetermined oxygen level threshold.

TECHNICAL FIELD

The present invention generally relates to a method for the production of pulverized coal, in particular for use in the metallurgical industry.

BACKGROUND

In the metallurgical industry, pulverized coal is generally injected as combustible into blast furnaces. It is important, in order to ensure good functioning of the blast furnace, that the pulverized coal is of good quality, i.e. that the pulverized coal has the right consistence, size and humidity level. The pulverized coal is generally produced in a grinding and drying installation, wherein raw coal is ground in a pulverizer and dried to the right humidity level before the resulting pulverized coal is fed to a hopper for storage or direct use in a blast furnace. It is known to subject the freshly ground coal to a stream of hot gas so as to dry the pulverized coal. The pulverized coal can e.g. be entrained by the hot gas from the pulverizer to a filter, where the pulverized coal is then separated from the gas and fed to the hopper. Part of the gas is recirculated and heated before it is reintroduced into the pulverizer.

For the correct functioning of the grinding and drying installation, it is important to monitor the oxygen level in the drying gas, generally downstream of the filter. If the oxygen level becomes too high, the combination of drying gas and pulverized coal may become an explosive mixture with potentially dangerous consequences. Generally, in the recirculation line, i.e. in the line returning the drying gas back to the pulverizer, exhaust gasses are extracted from the drying gas and fresh air is injected.

In known grinding and drying installations, the oxygen level in the drying gas is monitored and, if the measured oxygen level is found to be too high, the amount of fresh air introduced into the drying gas in the recirculation line is reduced. This allows lowering the oxygen level in the drying gas.

However, in some circumstances, e.g. if the raw coal is very dry and/or if the installation is run under reduced load, the reduction of the amount of fresh air introduced into the drying gas may not be enough to sufficiently reduce the oxygen level. Indeed, once the amount of fresh air introduced into the drying gas is reduced to zero, i.e. no more fresh air is introduced, the oxygen level may in such circumstances still be too high. In order to avoid any damage to the installation it may then be necessary to shut down the grinding and drying installation. Such a shut down not only leads to a loss of production, but also to extra costs relating to the replacement or conditioning of the drying gas.

BRIEF SUMMARY The Invention Provides an Improved Method for Producing Pulverized Coal, which does not Present the Drawbacks of The Prior Art Methods

More specifically, the present invention proposes a method for producing pulverized coal, the method comprising the steps of:

-   -   heating a drying gas, preferably an inert gas, in a hot gas         generator to a predefined temperature;     -   feeding the heated drying gas into a pulverizer;     -   introducing raw coal into the pulverizer, the pulverizer         grinding the raw coal into pulverized coal;     -   collecting a mixture of drying gas and pulverized coal from the         pulverizer and feeding the mixture to a filter, the filter         separating the dried pulverized coal from the drying gas;     -   collecting the dried pulverized coal for further use and feeding         the drying gas from the filter to a recirculation line for         returning at least part of the drying gas to the hot gas         generator;     -   determining an oxygen level in the drying gas, preferably in the         recirculation line, and comparing the determined oxygen level to         a predetermined oxygen level threshold.

According to a preferred embodiment of the invention, the oxygen level in the drying gas is determined during a grinding cycle wherein heated drying gas is fed through the pulverizer and raw coal is introduced into the pulverizer and if, during the grinding cycle, the determined oxygen level is higher than a predetermined oxygen threshold, water is injected into the heated drying gas before it is fed into the pulverizer, the volume of water injected being calculated so as to reduce the oxygen level below the predetermined oxygen level threshold. The injection of water into the drying gas during the grinding cycle allows increasing the overall volume of the drying gas, thereby reducing the relative oxygen volume. The water injection therefore allows reducing the oxygen level to an acceptable level and thereby avoids any damage to the installation or the need to shut down the grinding and drying installation.

According to a preferred embodiment, the method further comprises injecting, in the recirculation line, fresh air into the drying gas wherein, if the determined oxygen level is higher than the predetermined oxygen level threshold, the volume of fresh air injected into the drying gas is reduced.

Advantageously, the method comprises first reducing the volume of fresh air injected into the drying gas, and then, if the volume of fresh air injected reaches zero and the oxygen level is still higher than the predetermined oxygen threshold, injecting water into the heated drying gas before it is fed into the pulverizer, the volume of water injected being calculated so as to reduce the oxygen level below the predetermined oxygen level threshold.

Preferably, the predetermined oxygen threshold is chosen to be between 0 and 14 volume %, preferably between 5 and 12 volume %.

According to a further aspect of the present invention, the method comprises the further steps of determining an exit temperature of the mixture of drying gas and pulverized coal exiting the pulverizer; and controlling the exit temperature by controlling a volume of water injected into the heated drying gas before feeding it into the pulverizer. By controlling the amount of water injected into the drying gas upstream of the pulverizer, the temperature of the drying gas entering the pulverizer can be adjusted rapidly so as to take into account temperature differences occurring due to raw coal with different levels of humidity being introduced into the pulverizer. It is thereby possible to maintain the temperature of the drying gas exiting the pulverizer, hereafter referred to as exit temperature, as constant as possible.

The present aspect is of particular advantage during a startup phase of the installation, wherein the method comprises a startup cycle wherein heated drying gas is fed through the pulverizer without introducing raw coal, the exit temperature being kept below a first temperature threshold, and a grinding cycle wherein heated drying gas is fed through the pulverizer and raw coal is introduced into the pulverizer, the exit temperature being kept at a preferred working temperature. According to an important aspect of the invention, the method comprises:

-   -   during the startup cycle, heating said drying gas to a         temperature above the first temperature threshold and injecting         a volume of water into the heated drying gas, the volume of         water being calculated so as to reduce the temperature of the         heated drying gas to obtain an exit temperature below the first         temperature threshold; and     -   at the beginning of the grinding cycle, reducing the volume of         water injected into the heated drying gas so as to compensate         for the drop in exit temperature.

During a startup phase of the installation, drying gas is generally fed through the installation before raw coal is introduced into the pulverizer. This allows the individual components to be heated to the desired working temperature. By controlling the amount of water injected into the drying gas upstream of the pulverizer during this startup phase, the drying gas, which may be heated to a temperature above the maximum tolerated exit temperature, can be cooled down again so that the temperature downstream of the pulverizer does not exceed the first temperature threshold.

When the raw coal introduction is then started, a sudden drop in exit temperature occurs due to the addition of cold and wet material. By overheating the drying gas in the hot gas generator and subsequently cooling it through water injection, the temperature of the drying gas entering the pulverizer can be quickly adapted to the new operating conditions. A reduction of the quantity of injected water allows a rapid temperature increase of the drying gas entering the pulverizer so as to compensate for the temperature drop due to the introduction of the raw coal. As a consequence, the transition time, wherein pulverized coal is produced at lower temperature is considerably reduced or even avoided. The amount of unusable coal slurry is also considerably reduced, thereby increasing the efficiency of the installation.

The volume of water injected into the heated drying gas can be determined based on the exit temperature. Alternatively, the volume of water injected into the heated drying gas can be determined based on a pressure drop measured across the pulverizer. It is not excluded to use other measurements, alone or in combination, to determine the volume of water to be injected into the heated drying gas.

Preferably, during the grinding cycle and after compensation for the drop in exit temperature, the method comprises the further steps of reducing the heating of the drying gas; and reducing the volume of water injected into the heated drying gas to maintain the desired exit temperature. This allows reducing consumption of energy once the installation is running. Indeed, the importance of the overheating and subsequent cooling of the drying gas is particularly important during the startup phase of the installation, wherein it allows providing a buffer to compensate for the drop in temperature occurring when the introduction of raw coal is started. Once the installation is running, only smaller temperature drops might occur and the buffer can be reduced. During normal operation of the grinding and drying installation, there is hence no need to over heat the drying gas in the hot gas generator and subsequently cooling it to the working temperature.

In the recirculation line, part of the drying gas can be removed as exhaust gas. Apart from fresh air, hot gas can also be injected into the drying gas in the recirculation line.

The method may also comprise continuous monitoring of the exit temperature and comparing the measured exit temperature to a maximum temperature, wherein, if the measured exit temperature exceeds the maximum temperature, the volume of water injected into the heated drying gas is increased. This allows using the water injection means used for general process control, to be used for emergency cooling also.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more apparent from the following description of one not limiting embodiment with reference to the attached drawing, wherein

FIG. 1 shows a schematic representation of a grinding and drying installation used for carrying out the method according to the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a grinding and drying installation for producing pulverized coal using the method according to the present invention.

Such a grinding and drying installation 10 comprises a pulverizer 20 into which raw coal is fed via a conveyor 22. In the pulverizer 20, the raw coal is crushed between internal mobile pieces (not shown) or any other conventional grinding means into a fine powder. At the same time, a hot drying gas is fed through the pulverizer 20 to dry the pulverized coal. The drying gas enters the pulverizer 20 through a gas inlet 24. Upstream of the pulverizer 20, the grinding and drying installation 10 comprises a hot gas generator 26 in which a drying gas can be heated to a predefined temperature. Such a hot gas generator 26 is powered by a burner 27, such as e.g. a multiple lance burner. The heated drying gas is carried from the hot gas generator 26 to the pulverizer 20 via a conduit 28. As the heated drying gas passes through the pulverizer 20, from the gas inlet 24 to an outlet 30, pulverized coal is entrained. A mixture of pulverized coal and drying gas is carried from the pulverizer 20, via a conduit 32, to a filter 34, where the pulverized coal is again removed from the drying gas and fed to a pulverized coal collector 36, ready further use. The drying gas exiting the filter 34 is fed to a recirculation line 38 for feeding it back to the hot gas generator 26. The recirculation line 38 comprises fan means 40 for circulating the drying gas through the installation. The fan means 40 may be located upstream or downstream of a line 42, e.g. a stack, which is used to extract part of the drying gas from the recirculation line 38.

The recirculation line 38 further comprises gas injection means 44 for injecting fresh air and/or hot gas into the recirculation line 38. The injected fresh air and/or hot gas is mixed with the recycled drying gas. The injected fresh air allows reducing the due point of the drying gas and the injected hot gas is used to improve the thermal balance of the grinding and drying circuit.

According to an important aspect of the present invention, the installation 10 comprises water injection means 46 arranged downstream of the hot gas generator 26 and upstream of the pulverizer 20. The importance of the water injection means 46 will become clear in the description herebelow.

The water injection means 46 helps to regulate the dew point of the drying gas by regulating the oxygen level therein. In the recirculation line 38, part of the drying gas is extracted via the line 42 and fresh air may be injected via the gas injection means 44. In conventional installations, the oxygen level is monitored for safety reasons by means of an oxygen sensor 45 and, if the oxygen level is found to be too high, the gas injection means 44 is instructed to reduce the amount of fresh air introduced into the dying gas. A problem however occurs when the gas injection means 44 reaches its shut-off point, i.e. when the gas injection means 44 is completely turned off and no fresh air is injected into the dying gas. If the oxygen level is then still found to be too high, the volume of fresh air injected into the dying gas cannot be further reduced and a shutdown of the installation becomes necessary.

According to the present invention, the oxygen level in the drying gas can be reduced by injecting water into the drying gas by means of the water injection means 46. When the oxygen level measured by the oxygen sensor 45 is too high, the water injection means 46 can be instructed to increase the volume of water injected into the drying gas, thereby reducing the oxygen level downstream of the filter 34.

Preferably, the oxygen level is first reduced by the conventional method of reducing the volume of fresh air injected into the dying gas by the gas injection means 44 and if this is not sufficient, the oxygen level is then further reduced by increasing the volume of water injected into the drying gas by the water injection means 46.

Another function of the water injection means 46 may be to help regulate the temperature of the drying gas at the exit of the pulverizer 20. In operation, the drying gas is heated to a predefined temperature in the hot gas generator 26 and fed through the pulverizer 20. The temperature of the drying gas is reduced in the pulverizer 20 as the heat from the drying gas is used to dry the pulverized coal. The level of humidity of the raw coal determines the temperature loss of the drying gas. In order to prevent damage to the filter 34, the temperature of the mixture of pulverized coal and drying gas exiting the pulverizer 20, hereafter referred to as the exit temperature, is monitored, e.g. by means of a temperature sensor 48.

In order to maintain a correct exit temperature, the temperature of the drying gas entering the pulverizer needs to be controlled, which is generally achieved by controlling the output power of the burner 27 of the hot gas generator 26. Unfortunately this process has a relatively slow response time, meaning that once the installation has determined that the exit temperature is too high or too low and the burner 27 has been made to react in consequence, some time passes before the exit temperature reaches the correct exit temperature again.

The response time is particularly important during a startup phase of the installation. Indeed, initially, heated drying gas is fed through the installation before the raw coal is introduced. This allows the installation to heat up and reach the ideal working conditions. When, after a certain time, raw coal is then introduced into the pulverizer 20, the exit temperature suddenly drops well below the desired exit temperature. Conventionally, the burner 27 then reacts by further heating the drying gas so as to reach the desired exit temperature. The desired exit temperature is then however only obtained after a long delay and any pulverized coal obtained in the meantime may have to be discarded because it has not been sufficiently dried. Indeed, during a transition period wherein the exit temperature is too low, unusable coal slurry is generally obtained instead of dried pulverized coal.

According to the present invention, during the startup phase, the burner 27 is set to heat the drying gas well above the desired exit temperature. The heated drying gas is then subjected to controlled cooling by injecting water into the heated drying gas through the water injection means 46, whereby the drying gas is cooled so that the desired exit temperature can be achieved. After a certain heat-up time of the grinding and drying installation, when the raw coal is introduced into the pulverizer 20, the exit temperature suddenly drops well below the desired exit temperature. Instead of compensating for this sudden drop by adapting the heating temperature of the burner 27, the amount of water injected into the drying gas by the water injection means 46 is reduced. The heated drying gas is hence cooled less and the desired exit temperature can be kept stable. The reaction time of this procedure is considerably lower than the conventional one, thereby considerably reducing or avoiding a transition period wherein the exit temperature is too low and the production of unusable coal slurry.

It should be noted that this method shows its most dramatic advantages during the startup phase, i.e. during a transition period shortly after raw coal is initially introduced into the pulverizer. The present method is however also advantageous during normal operation of the installation. When a reduction of the humidity in the raw coal occurs, the exit temperature can be quickly brought back to the desired exit temperature should a sudden drop in temperature occur.

In order to optimize energy consumption, it is advantageous to gradually reduce both the heating and the subsequent cooling of the drying gas once the exit temperature has stabilized. If no such subsequent cooling is required, the water injection system can be switched off.

Advantageously, the water injection means 46 is also used for an emergency cooling. The method may comprise continuous monitoring of the exit temperature and comparing the measured exit temperature to a maximum temperature. When the measured exit temperature exceeds the maximum temperature, the water injection means 46 is instructed to increasing the volume of water injected into the heated drying gas, thereby reducing the temperature of the drying gas entering the pulverizer 20 and consequently also the temperature of the drying gas exiting the pulverizer 20. 

1. Method for producing pulverized coal, the method comprising the steps of: heating a drying gas in a hot gas generator to a predefined temperature; feeding the heated drying gas into a pulverizer; introducing raw coal into the pulverizer, the pulverizer turning the raw coal into pulverized coal; collecting a mixture of drying gas and pulverized coal from the pulverizer and feeding the mixture to a filter, the filter separating the dried pulverized coal from the drying gas; collecting the dried pulverized coal for further use and feeding the drying gas from the filter to a recirculation line for returning at least part of the drying gas to the hot gas generator determining an oxygen level in the drying gas and comparing the determined oxygen level to a predetermined oxygen level threshold wherein the oxygen level in the drying gas is determined during a grinding cycle, in which heated drying gas is fed through the pulverizer and raw coal is introduced into the pulverizer, and if, during the grinding cycle, the determined oxygen level is higher than the predetermined oxygen level threshold, a volume of water is injected into the heated drying gas before it is fed into the pulverizer, the volume of water injected being calculated so as to reduce the oxygen level below the predetermined oxygen level threshold.
 2. Method according to claim 1, wherein, in the recirculation line, a volume of fresh air is injected into the drying gas, and wherein, if the determined oxygen level is higher than the predetermined oxygen level threshold, the volume of fresh air injected into the drying gas is reduced.
 3. Method according to claim 2, wherein, if the volume of fresh air injected reaches zero and the oxygen level is still higher than the predetermined oxygen threshold, a volume of water is injected into the heated drying gas before it is fed into the pulverizer, the volume of water injected being calculated so as to reduce the oxygen level below the predetermined oxygen level threshold.
 4. Method according to claim 1, wherein the predetermined oxygen threshold is chosen to be between 0 and 14 volume %.
 5. Method according to claim 4, wherein the predetermined oxygen threshold is chosen to be between 5 and 12 volume %.
 6. Method according to claim 1, comprising: determining an exit temperature of the mixture of drying gas and pulverized coal exiting the pulverizer; and controlling the exit temperature by controlling a volume of water injected into the heated drying gas before feeding it into the pulverizer, the volume of water injected being calculated so as to bring the exit temperature to a preferred working temperature.
 7. Method according to claim 6, wherein the method comprises: a startup cycle wherein heated drying gas is fed through the pulverizer without introducing raw coal, the exit temperature being kept below a first temperature threshold, and a grinding cycle wherein heated drying gas is fed through the pulverizer and raw coal is introduced into the pulverizer, the exit temperature being kept at a preferred working temperature, wherein during the startup cycle, said drying gas is heated to a temperature above the first temperature threshold and a volume of water is injected into the heated drying gas, the volume of water being calculated so as to reduce the temperature of the heated drying gas to obtain an exit temperature below the first temperature threshold; and at the beginning of the grinding cycle, the volume of water injected into the heated drying gas is reduced so as to compensate for a drop in exit temperature.
 8. Method according to claim 6, wherein the volume of water injected into the heated drying gas is reduced at a rate determined by the exit temperature.
 9. Method according to claim 1, wherein the volume of water injected into the heated drying gas is reduced at a rate determined by a pressure drop measured across the pulverizer.
 10. Method according to claim 7, wherein, during the grinding cycle and after compensation for the drop in exit temperature, the method comprises the steps of: reducing the heating of the drying gas; and reducing the volume of water injected into the heated drying gas to maintain the preferred exit temperature.
 11. Method according to claim 1, wherein, in the recirculation line, at least part of the drying gas is extracted as exhaust gas.
 12. Method according to claim 1, wherein, in the recirculation line, fresh air and/or hot gas is injected into the drying gas.
 13. Method according to claim 1, comprising: continuous monitoring of the exit temperature and comparing the measured exit temperature to a maximum temperature; and if the measured exit temperature exceeds the maximum temperature, increasing the volume of water injected into the heated drying gas.
 14. Method according to claim 1, wherein the drying gas is heated in a hot gas generator powered by a lance burner.
 15. Method according to claim 1, wherein water is injected into the heated drying gas by means of a water injection device arranged between the hot gas generator and the pulverizer.
 16. Method according to claim 7, wherein the volume of water injected into the heated drying gas is reduced at a rate determined by the exit temperature. 